1. Field of the Invention
This invention relates to a water-soluble film forming composition and more particularly, a water-soluble composition for forming a film on a chemically amplified resist layer which lends itself to micro-processing technology in that it has high sensitivity to high energy radiation such as deep ultraviolet lights, electron beams and X-rays and is developable with alkaline aqueous solution to form a pattern.
2. Prior Art
To cope with the increased integration and speed of LSIs, chemically amplified positive working resist compositions using acid catalysts were developed as disclosed in U.S. Pat. No. 4,491,628 and 5,310,619, Japanese Patent Publication (JP-B) No. 27660/1990, and Japanese Patent Application Kokai (JP-A) No. 27829/1988. Because of high sensitivity, resolution and dry etching resistance, they are promising resist materials especially suited for deep ultraviolet lithography.
The deep-UV lithography, however, has the problem that processing of a resist layer to an accurate pattern size is difficult since a resist image is reduced in dimensional accuracy by the influence of standing waves associated with the use of monochromatic light and if a substrate has steps, by the influence of optical interference due to variations in thickness of the resist layer at the steps and the influence of halation at the steps.
Also chemically amplified resists suffer from the problem known as post-exposure delay (PED) that when deep-UV, electron beam or X-ray lithography is carried out, line patterns would have a T-top configuration, that is, patterns become thick at the top if the leave-to-stand or delay time from exposure to post-exposure baking (PEB) is extended. This problem, which arises probably because the resist surface is reduced in solubility, becomes a serious drawback on practical application. This not only makes difficult dimensional control in the lithographic process, but also adversely affects dimensional control in the processing of substrates using dry etching. In this regard, reference is made to W. Hinsberg et al., J. Photopolym. Sci. Technol., 6 (4), 535-546 (1993), T. Kumada et al., J. Photopolym. Sci. Technol., 6 (4), 571-574 (1993), and Hatanaka et al., Preprint of 1994 Spring Meeting of the Applied Physical Society, page 567, 29p, MB-2.
It is understood that the PED problem of chemically amplified resists is dependent on the environment, that is, basic compounds in the air largely participate in the PED problem. In the case of positive resists, light exposure generates acids at the resist surface which react with basic compounds in the air and are thereby deactivated. As the delay time until PEB is extended, more amounts of acids are deactivated and accordingly, decomposition of acid unstable groups are more unlikely to occur. As a consequence, an insolubilized layer is formed at the resist surface, resulting in a T-top configured pattern.
Several patterning techniques were proposed as having solved the above-mentioned problems associated with steps, for example, a multi-layer resist technique as disclosed in JP-A 10775/1976, an ARC technique (anti-reflective coating beneath resist) as disclosed in JP-A 93448/1984, and an ARCOR technique (anti-reflective coating on resist) as disclosed in JP-A 62520/1987, 62521/1987, 188598/1993, 118630/1994, and 148896/1994.
The multi-layer technique involves forming two or three layers of resist and transferring a pattern to form a resist pattern serving as a mask. Undesirably the multi-layer technique requires a number of steps and is low in production efficiency. Light reflection from an intermediate layer can cause a lowering of dimensional accuracy. The ARC technique is by etching an anti-reflective film formed beneath the resist layer. The dimensional accuracy is substantially lost by etching and the extra etching step lowers production efficiency.
In contrast, the ARCOR technique which involves forming a transparent anti-reflective film on a resist layer and peeling the film after exposure is able to form a fine resist pattern to high dimensional and alignment accuracy in a convenient way. By using low index of refraction materials, for example, perfluoroalkyl compounds (e.g., perfluoroalkyl polyethers and perfluoroalkylamines) as the anti-reflective film, the ARCOR technique of JP-A 62520/1987 minimizes reflection light at the resist layer/anti-reflective film interface, thereby reducing the variation of the pattern size of a resist image to one third as compared with a resist layer used alone.
However, since the perfluoroalkyl compounds are less soluble in organic solvents, they must be diluted with such diluents as Freon solvents in order to control the thickness of a coating film. Freon solvents are also used in peeling off anti-reflective films of perfluoroalkyl compounds. The use of Freon is now undesirable from the standpoint of environmental protection. An increased number of steps is another problem.
Using water-soluble polysaccharides as the anti-reflective film material, the technique of JP-A 62521/1987 eliminates intermixing at the resist/anti-reflective film interface and permits removal of the anti-reflective film to be done at the same time as the development step, offering a simple process. However, since the polysaccharides are not so low in refractive index as the perfluoroalkyl compounds, the variation of the pattern size is suppressed to only two thirds as compared with a resist layer used alone.
JP-A 188598/1993 discloses an anti-reflective film forming composition of a two component system comprising a film-forming polymeric binder which is soluble or dispersible in water or aqueous alkaline solution and a low refractive index fluorocarbon compound which is soluble or dispersible in water or aqueous alkaline solution. It is effective as an anti-reflective film material for conventional resists using diazonaphthoquinone compounds. Since all the fluorocarbon compounds used therein have an ammonium ion which can deactivate acid on the chemically amplified resist surface, the above-mentioned PED problem cannot be overcome.
JP-A 118630/1994 discloses an anti-reflective film forming composition comprising at least 90% by weight of a water-soluble film-forming composition and up to 10% by weight of a proton-generating material, based on the solids in the composition. Like JP-A 62521/1987, this composition also lacks the function as an anti-reflective film material for chemically amplified resists because the refractive index is not low. If water-soluble inorganic and organic acids which are exemplified as the proton-generating material are added in amounts of more than 10% by weight based on the solids, the pattern profile is deteriorated by the excess supply of acid. Ammonium weak acid salts which are also exemplified as the proton-generating material can deactivate acid on the chemically amplified resist surface, also failing to overcome the above-mentioned PED problem.
Finally JP-A 148896/1994 discloses an anti-reflective film forming composition comprising a polyvinyl pyrrolidone homopolymer and a fluorinated organic acid ammonium salt. It is used only with conventional resists. Like JP-A 188598/1993, this does not solve the PED problem of chemically amplified resists. Since this composition contains a polyvinyl pyrrolidone homopolymer and is thus free of a hydrophobic unit, a film of this composition can be formed on a resist film with difficulty and has a high refractive index.